CMRT Signal Processing & Antenna Theory 4 — Questions and Answers
Question 1: In radar signal processing, what is the purpose of the I/Q (in-phase/quadrature) demodulation scheme?
- To double the transmitted power
- To recover both amplitude and phase of the received signal (Correct answer)
- To eliminate range ambiguities
- To filter out the IF frequency component
Correct answer: To recover both amplitude and phase of the received signal
I/Q demodulation extracts both the in-phase and quadrature components, preserving full complex signal information including amplitude and phase.
Question 2: What is the relationship between radar range resolution and pulse width?
- Range resolution = c × PW (pulse width)
- Range resolution = c × PW / 2 (Correct answer)
- Range resolution = 2 / (c × PW)
- Range resolution = c / (2 × bandwidth)
Correct answer: Range resolution = c × PW / 2
Range resolution equals c×τ/2 where τ is the pulse width, because signals must be separated by at least half the pulse duration in range.
Question 3: Which type of antenna feed method for a parabolic dish results in feed blockage of the aperture?
- Cassegrain feed
- Prime focus feed (Correct answer)
- Offset feed
- Waveguide slot array
Correct answer: Prime focus feed
A prime focus feed places the feed element at the focal point directly in front of the dish, partially blocking the aperture and increasing sidelobes.
Question 4: Sea clutter in marine radar is most intense at which range condition?
- Long ranges with low sea state
- Short ranges with high sea state (Correct answer)
- Medium ranges regardless of sea state
- Long ranges with high sea state
Correct answer: Short ranges with high sea state
Sea clutter is strongest at short ranges where large sea waves produce intense backscatter returns that can mask small targets.
Question 5: In a phased array radar antenna, beam steering is accomplished by:
- Physically rotating the array
- Controlling the phase of each radiating element (Correct answer)
- Changing the carrier frequency
- Adjusting the transmitter power level
Correct answer: Controlling the phase of each radiating element
Phased arrays steer the beam electronically by applying progressive phase shifts across the array elements to change the direction of constructive interference.
Question 6: What is the purpose of a magnetron injection-locking technique in some marine radar transmitters?
- To increase peak power output
- To stabilize and control the magnetron's output frequency (Correct answer)
- To eliminate the need for a waveguide
- To reduce transmitter warm-up time
Correct answer: To stabilize and control the magnetron's output frequency
Injection locking synchronizes the magnetron's frequency to a stable reference signal, improving frequency stability for coherent processing.
Question 7: The antenna pattern's half-power beamwidth (HPBW) for a uniformly illuminated aperture of width D at wavelength λ is approximately:
- λ/D radians
- 0.886 × λ/D radians (Correct answer)
- 2λ/D radians
- D/λ radians
Correct answer: 0.886 × λ/D radians
The 3 dB beamwidth of a uniformly illuminated aperture is approximately 0.886λ/D radians, derived from the sinc function zeros.
CMRT Signal Processing & Antenna Theory 4
In radar signal processing, what is the purpose of the I/Q (in-phase/quadrature) demodulation scheme?